The invention is in the microelectronics field. The invention is applicable to devices that have circuits operating in synchronization with a reference oscillator, typically a crystal. The invention is particularly applicable to reduce voltage controlled oscillator phase noise in very high frequency phase locked loops.
A recent trend is toward the combination of oscillator driven devices into a single device. Wireless transceivers requiring an oscillator reference frequency might be combined with a computer or portable device having a highly different oscillator reference. In devices having multiple wireless communication functions, distinct communication functions may require highly different operational frequencies. For example, a cell phone has a distinct frequency range of operation, whereas a wireless local area network (LAN) or other network may have a considerably different range of operation. The combination of a LAN communication function and cellular network communication function may require the use of separate crystals for generation of separate operational frequencies. Similarly, a LAN may be combined with another microprocessor driven device, e.g., a computer, having a distinct reference oscillation frequency. As an additional particular example, Bluetooth and the IEEE 802.11 communication standards have been identified as being desirable to combine into computers, peripherals and devices having a different wireless transceiver function. The eventual market penetration of such low-end local area networks will be affected by transceiver price.
Bluetooth is a particular example having current market interest that is affected by price considerations. A Bluetooth transceiver is approaching the five dollar level. This renders the price of an external crystal for basing Bluetooth communications a significant expense. A cost reduction may be achieved by basing Bluetooth communications on another crystal reference offered by a device into which the Bluetooth transceiver is incorporated, e.g., a cell phone, a computer or a personal digital assistant (PDA). This allows the Bluetooth transceiver to share the same reference as the host device, and is particularly advantageous in wireless host devices. A wireless host device, such as a cell phone, is often sensitive to interference at the circuit board level from oscillator signals outside of its operational frequency. Accordingly, an additional potential advantage of having a Bluetooth transceiver share the host transceiver reference is the potential of a reduced interference with primary operations of the host device.
The required oscillation signal for a transceiver reference is typically generated by a phase locked loop (PLL). A phase locked loop controls an oscillator having less precision and stability than the reference signal to maintain a constant phase angle relative to a reference signal, e.g., a host crystal reference signal. Use of a high frequency oscillator permits the development of a transceiver reference signal at higher frequency than the host crystal reference signal.
The critical oscillator in a wireless transceiver, e.g., a Bluetooth or IEEE 802.11 transceiver, is the oscillator used to drive the radio frequency (RF) mixers. A number of local oscillator signals at particular frequency increments are generally required over a frequency range. The various required frequencies could be generated from any of the popular crystal frequencies by a fractional-N RF PLL, but flexibility required to accommodate the necessary crystal frequencies would translate into significant added circuit area and power consumption. An integer-N RF PLL is better from a circuit area and power consumption standpoint, but requires a reference PLL capable of generating a reference signal of a particular frequency, e.g., the frequency increments over the frequency range, with very little phase noise from any of the crystal frequencies.
The trend in wireless transceiver design is to integrate the whole system in CMOS technology. A suitable voltage controlled oscillator (VCO) in a CMOS PLL is a ring oscillator VCO since it avoids external components. Ring oscillator based PLL""s are typically noisy, though, and therefore present considerable design challenges. Accordingly, there remains a need for an improved CMOS phase locked loop.
The present invention addresses these and other issues, with a periodic controlled realignment of the ring oscillator VCO in a phase locked loop. A realignment to a buffered version of the reference signal is conducted periodically, at a time when an edge of the VCO waveform would ideally coincide with an edge in the reference signal.
A preferred embodiment CMOS phase locked loop of the invention uses a ring oscillator voltage controlled oscillator. A divide by M circuit is driven by an output of the voltage controlled oscillator. A control voltage circuit accepts a reference signal and a signal from the divide by M circuit, and produces a control voltage proportional to a phase difference between the output of the voltage controlled oscillator and the reference signal to control the voltage controlled oscillator. A realignment circuit responsive to the reference signal provides a realignment signal into the voltage controlled oscillator when an edge in the waveform of the voltage controlled oscillator ideally coincides with an edge of the reference signal.